The present invention relates generally to a full-color printing apparatus which is capable of forming a gray scale and, in particular, to a driving circuit for an electric thermo transfer apparatus wherein printing is effected on a recording medium by melting heat-fusable ink with the joules heat generated by the current flowing through a conductive sheet.
Various types of color hard copy apparatus have recently been proposed. Such color printing apparatus mainly utilize an ink jet method or a thermal transfer method which uses a thermal head. In both methods, the darkness or intensity of the printed image is determined by a quasi-area modulation such as dither matrix, for obtaining a full-color image. This is because the area of one dot is modulated stably only due to the presence of the color picture element. Thus, in order to maintain a resolution of the picture element to a certain extent, a recording head having extremely high density is needed, or a high density recording is required.
For example, when a (4.times.4) matrix is formed for representing 16 gray scale at 5 dot/mm of the density of the picture element, 20 dot/mm of resolution is necessary in the line head, while in the single head, the speed is reduced to onesixteenth of the case of printing only two levels. Accordingly, the thermal transfer method of the prior art is highly expensive and its speed is extremely slow.
U.S. Pat. No. 4,350,449 discloses a printing apparatus using the electric thermo transfer method as depicted in FIG. 24 of the present application. Referring to FIG. 24 of the present application, an ink sheet consists of a resistive layer 251, a conductive layer 252 and an ink layer 254 which faces a record medium 255. Therein, a printing electrode 256 and a return electrode 257 are in contact with the resistive layer 251 and a voltage is applied therebetween. As the current flows through resistive layer 251 and conductive layer 252, the joules heat is generated in a portion 251a of resistive ayer 251. This joules heat causes melting of the thermally transferable material in the contiguous portion 254a of ink layer 254 and thereby forms an image on record medium 255. The current flowing through resistive layer 251 concentrates on the portion just under printing electrode 256, since the current flows into conductive layer 252 in which resistance is lower than that of resistive layer 251. As a result, a print dot having the same area can be obtained. Although this type of printing apparatus has the advantage of high speed, it is necessary to form a matrix in the same manner as the method of using a thermal head. Consequently, in spite of the above advantage, the prior art provides a very expensive printing apparatus. See also U.S. Pat. No. 3,744,611 which is also directed to an electro-thermic printing device.